In the field of teeth restoration, metal copings are typically used in the construction of a dental crown and/or bridge. The metal coping functions as the understructure of the crown, and is typically covered with a coating of ceramic porcelain composition or a polymer-based veneering material. The metal coping supports the coating and provides the required structural strength and rigidity for the restored tooth to resist the forces of mastication.
There are methods in the art for the production of metal coping by casting it from a wax pattern, for example by the known “lost wax” technique. An accurate working model of the patient's teeth, which includes the preparation (die) of the tooth (or teeth) to be restored and its surrounding area, is used. As discussed in Kuwata et al., Color Atlas of Ceramo-Metal Technology (Ishiyaku EuroAmerica, Inc., 1986, pp. 96-101), a wax coping, which is an exact replication of the desired metal coping, is manually built by wrapping a thin sheet of wax around the working model of the preparation, and is adapted to the preparation surface. By using a heated carving knife, the margin of the adapted sheet wax is cut off and the sheet joints are bent and sealed by adding more wax. The wax margin is further processed in order to correspond to the margin line of the restoration, i.e. by additional carving and/or wax adding.
The finished wax-up of the coping is then removed from the working model and invested in a material that solidifies onto the external side of the wax coping and forms a mold (this stage is known as the “investment” stage). The combined structure is then heated such that the wax is burnt out, leaving a cavity into which the metal is cast. After hardening, the mold is removed from the metal casting.
Another method for producing metal coping involves a direct fabrication of the metal coping based on digital data. U.S. 2002/0137011 discloses an automated and digital method for the formation of the metal coping from a sheet of metallic material, which comprises: scanning a three dimensional image of the die of the tooth or teeth to be restored; digitizing the scanned three dimensional image into digital information, storing the digital information in a computer; feeding the digital information from the computer into a CNC (computerized numerical control) cutting machine; cutting out a section of material of metallic composition into a two dimensional configuration representing a two dimensional lay out of the scanned three dimensional image, adapting the cut out section of material over the die so that the material covers the die surface in close engagement therewith to form a single three dimensional structure having the shape of the die and heat treating the structure, into a coping, conforming in shape to the die.
WO 03/017864 discloses a method for producing a dental prosthesis such as a dental coping. The method comprises the following steps: obtaining three-dimensional digital data relating to a patient's dentition; designing a virtual prosthesis for the dentition using the three-dimensional digital data; transmitting digital data corresponding to the virtual prosthesis to an automated prototyping system; producing a prototype of the dental prosthesis with the automated prototyping system, the prototype made of a material that can be ablated; covering at least the prototype with a hardening material and removing the prototype from within said hardening material to produce a mold for the dental prosthesis; casting the dental prosthesis by filling the mold with a metal and removing the hardening material.
In the case of coping fabrication in the “lost wax” technique, be it the manual production or the automated, prototype production, the wax is used as it is easy to manipulate and allows a high level of accuracy. Typically, soft wax, i.e. wax with relatively high viscosity, is used. Special care must be taken in handling the soft wax, as it is very sensitive to physical stress. Indeed, any damage to the wax-up pattern before investment (i.e. during its production process or after) may lower the quality of the replicated dental coping.
Of general background interest, the following patents describe manufacturing methods for prostheses. In U.S. Pat. Nos. 4,663,720 and 4,742,464 an apparatus and method are disclosed for designing a prosthesis starting with a 3D model of the patient's dentition obtained by optical methods, and directly machining the prosthesis from a blank. In U.S. Pat. No. 5,092,022, a prosthesis or a negative mold therefor can be machined automatically from suitable materials, with the assistance of a computer which takes into consideration, inter alia, the shape of the zone of implantation previously obtained. In U.S. Pat. No. 5,452,219, a negative mold for a prosthesis is milled using a machine controlled by a program based on 3D data obtained from a tooth model. In U.S. Pat. No. 5,378,154 a method for machining a dental prosthesis is disclosed, wherein the outside visible part of the prosthesis is subjected to a material removal operation by means of a CNC machining tool which follow machining paths that follow 3D irregularly spaced curved lines. In U.S. Pat. No. 6,126,732, a shaped high-strength dental ceramic prosthesis is made by pressing a molding composition comprising 1-50 wt % glass particles and about 50-99 wt % ceramic particles to form a ceramic frame, veneering the frame and firing the coated frame. In U.S. Pat. Nos. 5,691,905 and 5,718,585 methods of milling and polishing a set of negative mold parts are disclosed. In U.S. Pat. No. 6,488,503, a process is described for producing an artificial tooth, in which polymerizable materials are injection molded into a mold in stages to produce a solid core applied on an inner layer, which is applied on an external layer of the artificial tooth. In U.S. Pat. No. 6,066,274, a device including an injection molding tool is disclosed for producing a sinterable ceramic and/or metallic product using engageable molding tool parts one of which is produced using a wet composition. In U.S. Pat. No. 5,382,164, a method of making restorations is disclosed, comprising: temporarily repairing a tooth area to be restored to a final shape, and taking a first impression of the tooth area to be restored; preparing the tooth to be restored, and taking a second impression within the first impression to form a physical model of the crown. This model is then scanned to provide an image thereof, and a computer uses this image to mill the final restoration.